This invention relates to gerotor devices which comprise a rotor having n lobes and which is located eccentrically and internally of a lobed annulus having a larger number of lobes (e.g. n+1). These parts form a series of chambers each bounded by lines of contact between the respective parts, and of different volumes: adjacent to a position where one rotor lobe is fully meshed between two annulus lobes the chambers are minimal, and at an approximately diametric position, (according to whether the rotor has an odd or even number of lobes) the chamber is maximal. When the rotor is rotated relative to the annulus the individual chambers vary in volume.
It is known from U.S. Pat. No. 4,863,357 to make both stator (a stationary annulus) and rotor conical, with variable pitch spiralling lobes which result in constant axial length chambers which however reduce in volume due to the reduced lobe height along the cones. This is to be used as a fluid compressor with flow essentially along the axis from the large end to the small end of the cone. Such a compressor requires a substantial axial length.
It is also known to make down-hole motors, that is axial flow pumps using the tubular stator with a spirally multiple-start internal thread, meshed with a generally cylindrical rotor having a multiple-start male thread differing in start number. Again the axial length is an important factor in pump output and flow is essentially along the axis from one end to the other.
Many gerotor designs are used as i.c. engine lubrication pumps in situations where compact axial dimensions are important and sometimes the inlet and outlet are to be at the same axial end. The spiralling arrangements with axial flow through the pump are then unsuitable for both reasons and the conventional arrangement for such pumps is to use a rotor and annulus which are both prismatic and moreover mount the annulus for rotation at a different speed to that of the rotor so that it is no longer a stator, which avoids the need for a wobble stick type drive. In such an arrangement there is a pulsating pressure output which can be part-smoothed by providing an outlet port extending over a substantial arcuate segment so that a series of such chambers is exposed to the outlet port.
There is a necessary clearance between the parts in a radial direction for example but not exclusively as so-called dirt clearance, but this is as small as possible to avoid leakage from high pressure chambers to lower pressure chambers, and ideally every chamber is always bounded by two lines of contact between the annulus and rotor.
There are two specific problems with pumps of this kind namely noise and pressure fluctuation or ripple. The former, noise, is due to clearance being taken up especially as the chambers go from the inlet side to the outlet side and vice versa, so that in practice as each rotor lobe moves to the lowest pressure position it tends to hammer on the annulus. The second problem is due to the succession of chambers moving into register with the ports and the pressure ripple is of greatest amplitude and lowest frequency with smaller values of n (and vice versa).
The object of the invention is to provide improvements.